Every offload tool says it verifies. The word covers three very different things, and the gap between them is the footage.
The three levels
- No check. Dragging a card into Finder or Explorer copies the files and tells you it finished. Nothing confirms the bytes arrived intact.
- Size check. The tool compares the file sizes on each side. Fast, and almost worthless: a file can be the right size and still be corrupt.
- Read-back. The tool re-reads every file from the destination drive and compares a checksum of those bytes to the source. This is the only one that proves the copy.
A size match is not a copy. It is a guess.
Why the read-back costs time
A real verify reads the whole file a second time, off the disk it was just written to. Full source-and-destination verification "typically takes about twice as long" as a size-only copy, in Hedge's own words. That second pass is the cost of knowing, and it is the cost worth paying before you format a card.
The trap: reading from memory
There is a quiet way to get this wrong. If a tool hashes the destination file straight after writing it, the operating system may serve those bytes from RAM cache, not from the drive. The check passes, a bad write goes unseen, and you get a false "verified".
A true read-back has to re-read the physical device, bypassing the cache. Stow does this on both macOS and Windows, which is the whole point of calling a copy verified.
The short version: if a tool only checks file sizes, or hashes from cache, do not trust it to clear a card. Verified means the bytes were read back off the disk and matched.
Sources
- Hedge OffShoot, Verification documentation (read-back, and the roughly 2x time of full verification)
- Imagine Products, Checksums and verification: speed vs security
Stow verifies by reading the disk
Not the file size, not the cache. Free for macOS and Windows.
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